1. Field of the Invention
This invention relates to the manufacture of printed circuit boards and, more particularly, to such boards having plated through-holes.
2. Description of the Prior Art
Printed circuit boards having plated through-holes are used extensively in electronic modules. The plated through-holes serve the purpose of establishing electrical connections between circuit components on opposite surfaces of the board. Generally such printed circuit boards having plated through-holes have been produced in the past either by a so-called subtractive or an additive process.
The subtractive process is more commonly employed. A copper clad substrate is utilized and holes are drilled at predetermined locations. Then the substrate is copper plated throughout including the inside surfaces of the holes. The thus copper plated circuit areas and the surfaces of the holes are masked in any conventional manner, for example, by photoresist and subsequent exposure and etching. Hence, when the mask is removed, the copper plated circuitry and the surfaces of the holes which are copper plated remain.
The subtractive process which has been described, for example, in the U.S. Pat. to Tanaka, No. 3,673,680, provides for good chemical adhesion between the original substrate and the circuitry. The reason for the good chemical adhesion is that the plating readily adheres to the clad. On the other hand, the clad is securely laminated to the substrate. However, difficulties have been experienced in this process in attempting to control the thickness of the plating as well as the smoothness of the plating surface. Moreover, uniformity of the plating and the chemical etching is not readily attainable.
The additive process is generally described in the U.S. Pat. to Ryan, No. 3,606,677. Here the circuitry is directly generated on an unclad substrate. The plating is only applied to precise areas defined by the photo-image, say between a photoresist layer. Hence, it is possible to provide strict dimensional control. The sidewalls of the photoresist prevent undesired growth of the plating. Therefore, the line width of the plating is precise enough to reduce the space required between adjacent conductors. This in turn means that the circuit density can be made much great. Since only the circuit areas are plated, the strict controls required for the chemical etching in the subtractive process to meet plating thickness and uniformity requirements are eliminated.
The inherent problem of the additive process is the lack of adhesion strength between the substrate and the plating. Claims have been made that this plating can produce sufficient adhesion strength, equal to that produced by foil lamination. However, this is not generally accepted by the industry and by the Armed Forces.
It will now be seen that each of the above-described processes for manufacturing printed circuit boards has certain inherent advantages; by the same token, they also have inherent limitations. A continuous effort is being made to achieve highdensity packaging of electronic components. This in turn requires the achievement of a reduction on the line width of conductors and concurrently of the space between conductors while still maintaining good adhesion strength between the substrate and the circuits printed thereon.